I'll have to look into it a little more. I believe, with Kunena, we drop an un-resized image into a directory, and it should resize it, but again, I'll have to investigate. What version of Kunena are you using? That will help us determine a fix. Mainly, need to know if it's 1.5 or 1.6, but the minor portion will help too (1.5.x or 1.6.x).
Thanks,
Alex